As above, it's more of a slicing movement. Sink one side and push downwards towards that rail, then increase pressure on the other rail to draw level. It's easier to do it when paddling at a bit of pace as well, to help break the water.

That seems to be what I'm doing. I usually put more pressure on my left side when going into the wave and balance it out with the right when I'm in the wave. I'm not sure I get completely under the wave though as there's usually some turbulence and I get knocked back a few feet.

When this is done properly is it like a shortboard duck dive where the whole board is submerged and you are completely under the wave?

How important is it to get your feet on the tail? My board is 7ft 3 and I struggle to duck dive massively as my feet are off the back and I cant get the nose down. I will definitely try the slicing maneuver.

Wow you can really see how far he gets under the wave. Pretty impressive!

Get nose of board into the water and the use your foot to get the rest of the board submerged. Definitely giving this a go in the morning! I foresee a lot of attempts before I get any sort of positive result! I suppose using this technique it's possible to duck dive any board.